1. Field of the Invention
The present invention rates to a cable network system constituted in a tree or star shape like, e.g., a bidirectional CATV (Cable Access TeleVision) system and an HFC (Hybrid Fiber and Coaxial) system and, more particularly to, a cable network system having a function of suppressing ingress noise in an upward transmission path.
2. Description of the Related Art
In recent years, a trend of changing CATV systems providing mainly video broadcast services into bidirectional systems and developing various bidirectional transmission services has been activated. The bidirectional transmission services include on-line services (to be referred to as PC on-line services hereinafter) for data terminals such as personal computers, real-time communications services for telephones, videophones, and the like, and VOD (Video On Demand) services for rapidly providing desired movie video and the like to users, as needed.
When the bidirectional services are to be realized in a cable network system having a transmission path branched in a tree or star shape, like the CATV system, measures against ingress noise are required.
More specifically, if there is a connector open terminal to which, e.g., no in-home device is connected in each subscriber residence, or if a used coaxial cable is insufficient in electromagnetic shielding characteristics though the in-home device is connected, interference radio waves such as shortwave broadcasts, and electromagnetic wave noise from electrical motors, such as a vacuum cleaner, or motorcycles flow in via the connector open terminal or the coaxial cable. FIGS. 27 and 28 show examples of data obtained by experiments on inflow noise. FIG. 27 shows the spectrum distribution of noise flowing from the connector open terminal. The inflow noise was observed in a band of 40 MHz or less. FIG. 28 shows the spectrum distribution of inflow noise in a state wherein a 5-cm lead line is connected to a connector terminal, on the assumption that a used coaxial cable is insufficient in electromagnetic shielding characteristics. More typical inflow noise was observed in the band of 40 MHz or less. In addition, FIG. 29 shows actual data obtained when a connector terminal was terminated by a terminator. In this case, inflow noise was not substantially detected.
If noise flows into respective subscriber residences in this manner, lots of noise merge on an upward transmission path to increase the level and be transmitted to a headend. This noise is generally called ingress noise. The ingress noise causes degradation of the transmission quality, and in some cases causes the system to fail in transmission.
FIG. 30 shows an example of the interference percent availability of ingress noise actually observed over several days in a bidirectional cable network having 1500 subscribers. As is apparent from FIG. 30, the availability of satisfying the C/I (Carrier to Interference) ratio of 10 dB per 1-MHz channel bandwidth was almost 100%. The availability of satisfying the C/I ratio of 24 dB as a high-quality transmission environment was 70 to 80% on average and was below 50% depending on the band. It was found from remaining observation results that the interference percent availability became lower as the channel bandwidth was narrower and the number of subscribers was smaller.
The ingress noise can be roughly classified into three types: narrowband coherent noise, broadband incoherent noise, and specific subscriber noise.
The narrowband coherent noise is electromagnetic waves having large transmission power, such as shortwave broadcast waves and military radio waves present in an upward frequency band (5 to 48 MHz in Japan, and 5 to 40 MHz in the United States). Although the bands are narrow, these electromagnetic waves flow from the connector open terminals and the like of almost all subscribers. If all signals which reach a headend are in phase, the noise level observed at the headend equivalently increases by 2(logS [dB] for the average inflow noise per subscriber where S is the number of accommodated subscribers. In fact, however, the noise level increases by about 14 logS [dB] because signals have a difference in propagation delay time therebetween.
The broadband incoherent noise is generated by strong electromagnetic waves radiated in the atmosphere from the sparks of an electrical motor and a gasoline engine, and discharge tubes and digital devices such as a personal computer. Although the frequency band is broad (2 kHz to 100 MHz), the noise level decreases by 1/f as a frequency f becomes higher. Noise flowing into the upward transmission path is not correlated with other noise (incoherent). The noise can be considered as Gaussian noise. For this reason, the noise level equivalently increases a. the headend by 10 logS [dB] for the average inflow noise per subscriber.
The specific subscriber noise is generated when a subscriber erroneously connects an amateur radio device or a digital device such as a personal computer to a cable, or when the subscriber intentionally sends an interference signal to the cable. Since this noise directly flows into the upward transmission path, the noise level may be kept high over a long time.
In addition to the above-described three types of noise, there is harmonic noise caused by signal distortion on the transmission path. The harmonic noise is caused by a nonlinear effect generated by corrosion of a fitting connector terminal on a trunk line cable. This noise can be prevented by proper maintenance and management of cable industrial companies.
Inflow portions of the ingress noise can be classified into two portions: a portion on a trunk system and a portion inside a subscriber residence. The trunk system generally uses a coaxial cable excellent in electromagnetic shielding characteristics. The ingress noise may flow from a loose connector or an old or worn cable. However, such inflow of noise can be prevented by proper maintenance and management of cable network system industrial companies. To the contrary, no measure is provided with respect to the noise flowing from the subscriber residence. The bidirectional services must be performed in consideration of this noise.
FIG. 31 shows a comparison of the spectrum distribution of ingress noise observed on an upward transmission and the assumed level of an upward data signal. Strong noise estimated to be narrowband coherent noise was observed around 6.5 MHz, 10 MHz, and 27 MHz, and noise estimated to be broadband incoherent noise was observed at remaining frequencies. It is supposed that a very-high-quality transmission path can be realized if both the narrowband coherent noise and the broadband incoherent noise are suppressed by more than 20 dB. However, this suppression cannot be achieved, so that the various measures are proposed as follows.
(a) HFC (Hybrid Fiber and Coaxial) Architecture
The HFC architecture aims at a reduction in ingress noise level by decreasing the number S of subscribers described above. The conventional CATV system broadcasts television video from a headend to several ten thousands of subscribers via only coaxial cables by using several tens of bidirectional trunk amplifiers. This CATV system is subdivided into a maximum of 500 home paths per subsystem by combining, e.g., optical fibers and coaxial cables, as shown in FIG. 32. Note that the home paths represent the number of homes to which cables are wired near the residences or under the eaves and services are immediately provided if the subscribers require them. The actual number of subscribers for the services is 60% on average in the United States, i.e., 300 subscribers per subsystem.
Referring to FIG. 32, a reception equipment for receiving television broadcasts sent via communication satellites, and an information transmission equipment for providing various bidirectional services, such as servers, routers, and switching units are installed in a headend (H/E) 1. A plurality of distribution hubs (D/Hs) 2, . . . are connected to the H/E 1 via exclusive optical fibers 3, . . . in a star shape. In the D/Hs 2, . . . , television broadcast signals and downward signals for various bidirectional services which are sent from the H/E 1 are modulated and then synthesized with each other. Thereafter, obtained signals are converted into optical signals and transmitted to fiber nodes (F/Ns) 5, . . . via optical fibers 4.
The F/Ns 5,. . . convert the optical signals sent from the D/Hs 2 into electrical signals, and transmit them to subsystems 20 each having 500 home paths. In each subsystem 20, trunk line coaxial cables 6 (to be referred to as trunk line cables hereinafter) are connected around the F/N 5 in a tree or star shape. Tap-offs 8 are arranged on these trunk line cables 6 to branch the trunk line cables 6 into drop coaxial cables 9 (to be referred to as drop cables hereinafter). The drop cables 9, . . . are dropped in subscriber residences 10.
In the subscriber residences 10, as shown in FIG. 33, a television receiver 11 can be directly connected to an in-home splitter 18 to receive television broadcasts, and a television receiver 13 can be connected to the in-home splitter 18 via a set-top box (STB) 12 to receive the VOD services. In addition, a personal computes 14, a videophone 15, or a telephone 16 is connected via a modem to receive the PC on-line services and the rial-time communications services. Bidirectional trunk amplifiers 7 for compensating the attenuation of signals are arranged at a plurality of portions on the trunk line cables 6.
While the VOD services and the PC on-line services are received, upward signals transmitted from the STB and the modem are sent to the F/N 5 via the drop cables 9, the tap-offs 8, and the trunk line cables 6. The signals are converted into optical signals by the F/N 5, and the optical signals are sent to the D/H 2 via the optical fiber 4. The sent signals are converted into electrical signals by the D/H 2 and then demodulated. The resultant signals are sent to the H/E 1 and processed. Upward and downward signals are transmitted on the bidirectional transmission path using the trunk line cables 6 and the drop cables 9 in such a manner that they pass through different frequency bands.
In general, the cable network system using the HFC architecture has a service area of a maximum of about 50 km per headend. This cable network system can accommodate a maximum of 300,000 home paths. The distribution hubs are arranged at a maximum of about 15 portions. Therefore, one distribution hub deals with a maximum of 20,000 home paths. A maximum of 40 subsystems are connected to the distribution hub. However, since downward signals include analog video signals, a very expensive laser diode having high lineality is required to convert electrical signals into optical signals. For this reason, in many cases, identical signals are transmitted in the downward direction to five subsystems as a unit, i.e., to every 2,500 home paths (the average number of subscribers: 1,500).
To the contrary, upward signals are basically operated for each subsystem because of ingress noise suppression. However, to reduce the cost of a demodulator for an upward signal set in the distribution hub, the upward signals must be operated for 2 or more subsystems as a unit in fact.
FIG. 34 shows the degree of improvement by the HFC architecture when the operation unit in a conventional system not using the HFC architecture is set to 50,000 home paths. From FIG. 34, it was found that the HFC architecture was an effective method to reduce the ingress noise. However, as is apparent from the observation result (observation in 5 subsystems) in FIG. 30, the influence of the ingress noise is still large even with the HFC architecture in terms of the transmission quality. For this reason, parallel use of the following various measures has conventionally been examined.
(b) Frequency Agility
The frequency agility is a method of switching a specific frequency band to another frequency band when the transmission quality is degraded in the specific frequency band. This method is effective for avoiding the influence of a strong interference wave caused by the narrowband coherent noise or the specific subscriber noise. However, this method cannot provide an essential solution such as the reduction of the ingress noise itself.
(c) Low-Efficiency Modulation Scheme
The signal-to-noise ratio is low on the upward transmission path where the ingress noise is present. For this reason, it is difficult to employ a high-efficiency modulation scheme such as QAM in which signals are symbolized and modulated in units of 4 or 6 bits. In fact, a modulation scheme such as QPSK is employed at most.
(d) Error Correction/Retransmission
Error correction/retransmission is a method of estimating and correcting an error portion when received data have a bit error due to the ingress noise or the like, and if the error portion cannot be completely corrected, requiring retransmission to a transmission source by a communication protocol such as a TCP/IP (Transmission Control Protocol/Internet Protocol). However, this method cannot provide an essential solution for the ingress noise, either. In addition, the transmission efficiency decreases due to an error correction code added to actual data or retransmission.
(e) Band Reduction+Frequency Division Multiplex
Band Reduction+frequency division multiplex are a measure which pays attention to a reduction in influence of the ingress noise by narrowing the channel bandwidth, as described above, and tries to effectively use frequency bands as much as possible by using bands free from interference waves generated by, e.g., the narrowband coherent noise or the specific subscriber noise. It is designed to ensure a desired transmission capacity by determining a band for each channel and increasing the number of frequency carriers, i.e., by applying a frequency division multiplex scheme. However, this measure car,of provide an essential solution for the ingress noise, either, similar to the above-described schemes.
(f) Bridger Switch
Bridger switches are arranged in units of trunk line cables branched from, e.g., a fiber node. When strong ingress noise is observed, the bridger switches are sequentially turned off to specify a trunk line cable to which the generation source of the noise is connected, and to cut the trunk line cable from the system. This scheme is effective for the specific subscriber noise. However, services for all subscribers connected to the cut trunk line cable are stopped. In addition, human-wave tactics must be employed to search the subscriber as the generation source on the specified trunk line cable. This may lead to a serious problem when the bidirectional services get into stride in the future, and when the bridge switches must be often operated due to the carelessness of subscribers.
(g) High-pass Filter
According to a method using high-pass filters, when the HFC architecture is realized, high-pass filters which pass only signals in a downward transmission band therethrough and cut off signals in an upward transmission band are attached to all subscriber residences except for the residences of subscribers who desire the bidirectional services. With this arrangement, only the subscribers who desire the bidirectional services can use the upward transmission band. This method is effective when the number of subscribers who desire the bidirectional services is small. However, as the number of subscribers who desire the bidirectional services increases, the inflow amount of the ingress noise increases. If the subscribers who desire the bidirectional services are 1% of all the subscribers, the inflow in amount can be effectively reduced by 20 to 28 dB; if 20%, it is reduced by only 3 to 4 dB.
(h) CTU Scheme
In the CTU (Coaxial Termination Unit) scheme, services are divided on the frequency band into services requiring a broadband for transmission in the upward direction, such as the PC on-line services and the real-time communications services, and services capable of using a narrow upward transmission band, such as the VOD services. As for the former, as shown in FIG. 33, a CTU 17 is arranged at a position before a subscriber residence to terminate a cable, thereby preventing the inflow of noise. In an actually proposed scheme, a frequency band of 10 to 40 MHz is assigned to the former, and a band of 5 to 10 MHz is assigned to the latter, as shown in FIG. 35. The CTU 17 incorporates a modem function for the PC on-line services and the real-time communications services. The filter characteristics and the like are designed not to flow noise to the band of 10 to 40 MHz from a subscriber residence.
As for the latter, the cable is directly connected to an STB or a television receiver via an in-home splitter arranged in the subscriber residence. For this reason, noise flows into the band of 5 to 10 MHz from all subscribers. However, since the transmission rate is low, and the transmission band is narrow, the influence of the ingress noise is relatively weak.
In general, the expensive CTU 17 cannot be set for a subscriber who does not desire the bidirectional services. For this reason, in the CTU scheme, even if the CTUs 17 are set for 20% of the subscribers, the ingress noise is reduced by only about 1 dB. For this reason, a remarkable effect cannot be expected until the installation ratio of the CTU 17 increases to about 100%.
Further, a scheme using the CTU 17 in combination with the high-pass filter can be considered. That is, the high-pass filter is attached for a subscriber who does not desire the bidirectional services, and the CTU is set for a subscriber who desires the bidirectional services, instead of the high-pass filter. According to this scheme, the ingress noise does not flow in the band of 10 to 40 MHz. In fact. however, the CTU scheme is not practically used because of the following various problems.
More specifically, the first point is how to supply power to the CTU when the CTU is attached outside a residence, e.g., under the eaves. In telephone services using the cable system (to be described later), power is supplied to a telephone modem at an AC voltage of about 100 V via a drop cable due to the necessity of the supply of power to a telephone. It is not economical or practical to supply power looking via the coaxial cable ahead to the PC on-line services requiring higher speed operations and new services to be provided in the future, in addition to the telephone services. Therefore, power must be supplied from a commercial AC power supply to the CTU. However, a work for a new power supply of the CTU set outside the residence is undesirable because it complicates the work and results in an increase in work cost.
The second point is how to perform an in-home wiring work for each service. One of the features of the bidirectional cable system is to use a shared medium, i.e., a single transmission medium for various purposes and application purposes. To perform the in-home work for each new service is to increase an economical burden on a subscriber, interrupting invitations to the new services.
The influence of the ingress noise on the upward transmission path and the prior art described above are described in detail in the following references: [1] Cable Labs, "Two-Way Cable Television System Characterization, Final Report", Apr. 12, 1995. [2] C. A. Eldering, et al., "CATV Return Path Characterization for Reliable Communications", IEEE Communication Magazine, pp. 62-69, August 1995.
As described above, various measures have been proposed. Some of these measures are accompanied by enormous investment, like the HFC architecture. Even if these measures are performed, the ingress noise cannot be essentially removed. If there is provided a low-cost and practical solution to sufficiently suppress the ingress noise, a great effect will be attained.